1. Field of the Invention
The present invention relates to a hydraulic power steering apparatus suitable for use in automobiles and the like.
2. Discussion of the Prior Art
There has been a power steering apparatus which is designed to increase the total rigidity of a steering wheel around the neutral position by preventing the increasing of the differential pressure across a power cylinder. Such power steering apparatus is composed of an engine-driven pump 1, a reservoir 2, a power cylinder 3 for assisting the steering operation, and a rotary control valve 5, as shown in FIG. 1. The rotary control valve 5 fluidly controls pressurized fluid which is supplied from the pump 1 to the power cylinder 3, upon rotation of the steering wheel (not shown). The control valve 5 is composed of a first control portion 6 having variable orifices 6A-6D of center-open type, and a second control portion 7 having variable orifices 7A and 7B of center-closed type communicating with the pump 1 and variable orifices 7C and 7D of center-open type communicating with the reservoir 2.
In the above-mentioned constitution, since the center-closed variable orifices 7A and 7B remains closed around the neutral position, the differential pressure between both fluid chambers 3A and 3B is maintained to be substantially zero. As the steering wheel is turned beyond a certain angle from the neutral direction, for example, in rightward, the center-closed variable orifice 7B communicating with the pump 1 begins to open, and the opening area of the variable orifice 7D communicating with the reservoir 2 is reduced, so that the differential pressure is generated between the both fluid chambers 3A and 3B to assist the steering operation.
In this manner, the differential pressure between the both fluid chambers 3A and 3B of the power cylinder 3 can be prevented from increasing around the neutral position, whereby the total rigidity of the steering wheel around the neutral position can be increased.
However, such power steering apparatus involve some drawbacks. When the steering wheel is returned to the neutral position, the rotary control valve 5 comes back to the neutral position. In such a state, flow passages leading to the both fluid chambers 3A and 3B are closed by the center-closed variable orifices 7A and 7B. Although the pressurized fluid is supplied from the pump 1 to the fluid chamber 3A low in the pressure through the center-open variable-orifices 6B, 6D and 7C, such supply is delayed due to the resistances of the variable orifices 6B, 6D and 7C, which causes the delay in the returning of the steering wheel to the neutral position.
Further, it is recognized that power assistance at a low vehicle speed may be desirably more than that at a high speed and is required even around the neutral position of the steering wheel. But the conventional control valve 5 has a fixed hydraulic characteristic. Namely, the differential pressure across the power cylinder 3 is changed in dependence upon the rotational angle of the steering wheel, and has no relation with the vehicle speed. As a result, the total rigidity of the steering wheel is maintained to be high around the neutral position and therefore, a suitable power assistance can not be obtained in accordance with the vehicle speed.